A two-dimensional numerical model is employed to investigate the perturbation of time-varying electromagnetic fields by a subducting lithospheric slab. A simple thermal conduction model, a model with shear strain heating, and a model which takes into account the production and upward movement of partially melted material from the top of the slab are considered for two periods. Also, a continental/oceanic interface is included. The results indicate that the hot material above the slab considerably affects the spatial behavior of the electromagnetic field components at the surface. The maximum perturbation of the vertical magnetic component occurs over the point where subduction begins in the simple conduction case, but when shear strain heating is present, the maximum perturbation in this component occurs on the continental side of subduction. The nature of the vertical magnetic field perturbation changes considerably when rising melt is present. Apparent resistivity profiles across the subduction zone are substantially different for the three models. The calculated apparent resistivity is significantly reduced when partial melt is present.